Waihi Dredging Plant

Extracts from the book by Colin Townsend, 2003 (with kind permission).


The Ohinemuri River, "River of Gold" is found in the Waihi Ward in the district of Hauraki and flows generally in a Westerly direction from its headwaters, an insignificant gully where water runoff from the surrounding hills is sufficiently concentrated to be seen. Its origin is not a fixed spring so the exact location of the discernible river will vary depending on rainfall and may be further up or down the gully at any point in time. It gathers more in magnitude where the Mataura and Walmsley Streams meet and after meandering past Waihi the "Town of Gold," where in 1903 the first gold extraction plant operated, the river swirls its way beyond the ruins of the grand Waikino Battery and the derelict mines of Owharoa. Then through the towering Karangahake Gorge passing the now historic sites of the Woodstock and Talisman mines situated on the left bank of the bolder strewn Waitawheta Stream. The lofty ruins of the Crown Battery look down on the clear waters of the Ohinemuri as it exits the township of Karangahake on its journey to Paeroa "World Famous in New Zealand." The river then passes the historic site of the Waihi Paeroa Gold Extraction Company Limited disbanded in 1918, before finally discharging its waters in to the Waihou River, a journey of approximately sixty kilometres.

This story tells of dredging for gold in the North Island in the Ohinemuri River from 1897 until 1918. It concentrates in the main on two sites, Waihi and Paeroa. Of the total of four North Island sites where dredging took place the remaining two were at Coromandel and Thames where foreshore dredging was engaged. This story exists because certain events happened. First the initial discovery of gold in the region. Then in 1895 a government proclamation was made. The new improved process of gold recovery using cyanide as opposed to mercury becoming available in 1899; the inventing of conical air agitators and improving the effectiveness of tube mills plus the at times frustrating exploratory experiments of all these components by the Ohinemuri River Syndicate, formally known as the Waihi Dredging Co.

My thanks are extended to many but especially the Paeroa Museum, Paeroa Library and their staff who were always patient and helpful. My wife Patricia, Mr. Eric Lens and Mr. Eric Coppard, both of Waihi, Mr. Bruce Judd of Thames and Mr. Lud Sparks Paeroa for his editing skills.

So let the story be told…

Chapter One, Gold is discovered.

The discovery of gold in payable quantities at Waihi in 1878 followed by Waikino, Owharoa and Karangahake meant many men were employed working the numerous claims. The work meant long hours of hard and dangerous work, which saw many accidents, some resulted in fatalities. The gold was not won easily for it was embedded in quartz and for the mine owner’s extraction of the payable bullion was a major problem. In the early 1890s mining equipment and techniques used were basic and labour intensive. The first need in the process of the gold recovery was to "soften" the ore before it was crushed by the heavy stampers. The ore was taken from the mine to the drying kilns/burning pits where it was partially roasted. The accepted rule for the "roast" was a ton of wood for each ton of ore.

This "roasting" process while drying the ore would also cause oxidation of the sulphides and this made it easier for the divided gold and silver to form an amalgam (combination) with the mercury process when the crushed ore was mixed with it. The main advantage of "roasting" was the ease which the dry ore could be reduced to a fine dirt in the stamp mortar-boxes. Burning pits/kilns were excavated near or at the mine and handy to the battery with the narrowest point at the bottom of the pit. Some pits/kilns held up to fifty tons of ore. The kilns or pits were charged by first having a layer of firewood in the bottom set on end round the side of the pit then a layer of ore was placed then more firewood then more ore and so it went until the pit was full. After a "roast," trucks would take the burnt ore to the crusher, which would in eight hours break down forty to fifty tons of ore. The ore would pass from the crushers to the stamp-hoppers and be broken down further still. The remains of the crushed ore after processing were dumped into the Ohinemuri River. This rubbish was at times reported as still having bullion content as high as 60%. If one multiplies the dumping from all the mines in the district times twenty- four hours a day seven days a week it is easy to see that the river was being filled with so much golden waste that it was indeed becoming a river of gold.

Chapter Two, The Mining Act of 1891.

Parliament and the Mining Act of 1891 created the next circumstance for the existence of gold dredges in the North Island for in July of 1895 the Governor of New Zealand, Lord Glasgow, had proclaimed the Ohinemuri River to be a sludge channel. This proclamation now legally gave the mines the right to be able to dump as they had been doing for some time any and all rubbish including poisons like mercury residue into its waters. Farmers, business people and residents of the districts of Waikino through to Paeroa were agitated and concerned for they could see the river becoming clogged with all sorts of mining debris and "waste" tailings.

The sides of the river were closing in as well as the bottom silting and rising. After heavy rain when runoff water was gathered in the Ohinemuri River from swollen feeder streams the excess water would rush unimpeded to the Waihou River. But now with the river having a restrained area of flow, flooding was a reality and trapped excess contaminated water was finding its way into crops, buildings, businesses and homes of the people.

Chapter Three. Cyanide process. Air Agitation Tanks. Tube Mills.

Gold to be recovered by the cyanide process discovered in 1894 required three things to happen.

In the presence of air, gold is dissolved in a solution of sodium cyanide.

The main minerals in the ore are not attacked by a too strong solution of cyanide.

The dissolved gold can be solidified from the solution by using what was then cheap and readily available metallic zinc or zinc dust.

The mine owners were quick to grasp the significance of this, while using the mercury method they were only recovering at best 40 - 60% of bullion; using the cyanide solution the recovery rate was as high as 98%. This was the entire simple chemistry lesson and result that the owners and managers needed. From 1902 until 1904 at the Waihi site of the Ohinemuri River Syndicate, it is thought the proprietors; two men named Mr. F. C. Brown and a Mr. Thompson after much experimentation had perfected the building of air agitation tanks. The building of these tanks was not a large problem but the cyanide process would not work unless the ore dissolving in the solution had air and was continually moving. The two men finally found a simple process of using pumps connected to the base area of the tank through which compressed air was forced which kept the solution moving and full of the much needed air.

Tube mills or revolving horizontal crushers were about 20 feet long and up to 6 feet in diameter and were made from very heavy riveted steel plate with flanges at each end. These flanges to keep the mills in place had to be bolted to heavy end plates. At one end of a tube mill there was a space to feed in the mixture of ore and water, which while being crushed would wind its way to the discharge opening at the other end of the mill. Large spur wheels drove the tube mills from a motor fitted with friction clutches because of the tremendous torque necessary for starting to turn such a heavy mass of metal and ore.

Now that the necessary components for the telling of the story of the Waihi Paeroa Gold Extraction plant have been established and in brief been explained we can move on. So from the discovery of gold let us move to the successful recovery of gold?

Chapter Four, Foreshore Dredging Coromandel and Thames.

I felt that in a book such as this a brief look at the dredging operations that took place at Coromandel and Thames was warranted. The claim at Coromandel was on the foreshore of the harbour and it was first known as the Longford’s syndicate but was later taken over by an English company, Armstrong Gold-dredging Company Limited with Mr. S. L. Wilson as the Dredge-master using a dredge transported from the South Island. It became operational on the 8 December 1911 and worked to a depth of 35 feet and removed 100 cubic yards of material per hour.

The Thames Foreshore Dredging Works had Mr. H. H. Adams as its supervisor and had worked the foreshore continuously during the year of 1910. These sands were sent to the Moanataiari battery for cyanide treatment. Several men worked 7,622 tons for a return of £1,195. For the year ending 1911 the company with ten men employed had again dredged their claim but the returns were poor. For 9,000 tons of tailings treated at Moanataiari the return was only £882 pounds. In 1912 the dredge worked for only half the year for a small return of £299. A poor return indeed but it did help to make a total bullion return of 4,283 ounces.

The Judd Plant that was on the north side of Burke Street Thames had been thought of, as a dredging plant but this has not proven to be so. Three brothers, William, Walter and James Judd worked and processed tailings but these had been previously stacked on dry land. Only picks, shovels and wheelbarrows required here. There was definitely not a dredge used in the recovery of 6,861 ounces of bullion.

Chapter Five, The Ohinemuri River Syndicate.

The Waihi Dredging Co. in January of 1897 was formed by Mr. E. H. Barber who had been successful in England in forming the syndicate with a capital of £5,000. Some time later buildings were under construction on this Waihi site on the banks of the Ohinemuri and Waitete stream and a simple dragline was used to recover tailings. Subsequently the plant changed hands to Mr. Brown and Mr. Thompson and they started operation in 1903 as the Ohinemuri River Syndicate continuing through to 1908 when it changed hands once more, becoming the Waihi Paeroa Gold Extraction Co.

When I visited the original site in 1992 it was hard to imagine a gold extraction mill being there at all. The day was sunny and warm and the place very quiet, lush and green with spreading shade from a tree or two, it was a perfect place for a picnic. Sipping from a flask of tea I let my imagination take over and it was not long before I could hear the chug a chug of the little suction dredge and the clanking of the dragline/bucket elevator lifting from the river the gold bearing tailings. These in turn being noisily crushed in the greedy tube mills before finding their way into the vertical air agitation tanks. With the bullion extracted it was only a small trip for the now treated tailings to dumped back into the river.

Today there remains very little of the plant. To those visiting the site for the first time it may appear to be just of some worn and meaningless broken concrete foundations, and some other ruins used by the farm owner as a shelter to store bales of hay.

Back in 1903 the first year of its operation as the Ohinemuri River Syndicate, the syndicate with good fore site had taken up nine river claims from the Ohinemuri junction with the Waihou River to its operating site near Waihi. The syndicate after treatment was anticipating a return of ten shillings per ton of tailings. The accountants thought after paying expenses including wages for several workers there would be a small margin of profit.

The syndicate to obtain its mining license had to agree to certain conditions imposed on them by the gold fields Warden. They had to ensure their operations would not damage in any way the banks of the river or cause damage to the near- at- hand landowners. The Warden having at all times the power to impose further conditions on the syndicate if he deemed fit to do so. This new undertaking of 1903 evoked confidence with the cyanide process, and improved agitation tanks the formula for success was there. They only had to get it right; but try, as they might they were unable to realise their dream.

Like all new ventures and experiments fine-tuning was required. In fact a lot of fine-tuning. The reality being that the work done in 1903 after spending more than £7,000 pound had proved to be unsatisfactory and the further expense of a second plant was required. It was decided that it was necessary to possess a more effectual method of grinding the tailings. Early in 1904 was erected a small tube or flint mill. A second hand rotary roasting furnace was purchased and four sections of it converted in to a mill. This proved to work as a most efficient grinding machine being able to finely grind up to a 100 tons of tailings per week, which would then pass through a 200-mesh sieve. (Each square inch was made up of 40,000 holes.) They then faced a serious difficulty in the treatment of this now fine material in the cyanide tanks as perfect agitation being necessary to effect a high extraction of bullion.

Later in the year and after considerable work, trouble and expense with various agitation tanks the proprietors Mr. Brown and Thompson invented a system that was superior to any other system of that time and patent-rights were applied for. 2000 tons of tailings were treated with this new process and the extraction of bullion was found to be most satisfactory. The plan was to increase the capacity of the present plant to 50 tons per day and to erect another larger plant capable of dealing with a 100 tons a day at Paeroa. The target date set for completion of this work was by the end of 1905. With this in mind Mr. Brown returned to England to raise further capital for this purpose.

1904 the first year of operation with 10 men employed showed a return of £884; while a report in 1905 finds the Syndicate stating that all their previous problems had been successfully overcome. A year later showed that the extraction of bullion had, thus far, not been commercially successful enough to meet the working costs of the plant. This was most disappointing to the owners who had continuously spent large sums of money to ensure a profit.

It was about this time an Australian syndicate showed interest in the Ohinemuri River and sent to the district a representative for checking and testing the gold reserves in the claims held by the Ohinemuri syndicate. It became obvious the claims were of good value but Australia decided not to proceed with the venture of taking the plant over.

It was becoming public knowledge that the Ohinemuri River Syndicate was still in trouble. It was now being reported that the expected tonnage of treated tailings was not being met.

It was about this time in 1906-07 that Mr Percy Nichol Kingswell and his officers started taking soundings and making experiments with the tailings found from the junction of the Waihou River for the nine miles of claims up the Ohinemuri River. In the years 1907 and 1908 only a small amount of work had managed to be done. On a positive note about October 1908 the syndicate again changed hands.

The now Waihi Paeroa Gold Extraction Company with new capital installed new machinery in the plant. This consisted of four tube mills, agitators, concentrators and a new air compressing plant.

Some of the new owners’ of the Waihi Paeroa Gold Extraction Company were Mr. P. N. Kingswell, chairman. (He was also chairman of the Portland Cement Company.) Director’s were Colonel W. D. Holgate, Auckland, Mr. R. B. Jackson, Nelson, and Mr. D. W. Duthie with Mr. W. R. Holmes, auditor. Capital raised was £100,000 £1 shares. With him was his associate and manager Mr. Robert Mark Aitken. Both were enthusiastic and confident of better success because they were sure they could increase the amount of tailings being treated and thus increase the bullion recovered. With the problems of crushing the tailings and the new cyanide process now under control this new company was most confident of success.

Mr R.W. Aitken Manager had been educated at the Thames School of Mines where he obtained first class certificates in surveying, mining, battery superintendent and metallurgy. Prior to his position with Mr. Kingswell they had been on the West Coast of the South Island where during a stay of eight and a half years Mr. Aitken was a lecturer to the School of Mines at Reefton. He was a metallurgist to a number of companies as well as connected to bucket dredging. He had also been the battery superintendent for the Alpine Tailings Company at Lyell.

Meanwhile M. Kingswell had been granted a special dredging claim at Matakitaki just out of Murchison. It was on the West Coast that he with Mr. Aitken were first to make the cyanide process a commercial success.

On his arrival at the Waihi site Mr. Aitken took charge and supervised the rebuilding and installing new machinery costing £16,000. This took nine months to complete and the plant operated for eighteen months treating a 100 tons of tailings daily experimenting with their fineness while grinding and treatment. From this it was proved economical to run such a plant and the intention firmed of building a larger plant at Paeroa.

At this site air suction dredges used Pohle air lifters with water jets assisting the lifting of the tailings from the river. The tailings were lifted five feet above water so they could be discharged into barges. The mix was one of water to one of tailings. Excess water overflowed from the barge before being taken to the works for unloading by bucket elevator. The dredge required eighteen inches of water to float while the loaded barges required four feet. The tailings when taken from the hopper would be cleaned in vanners, screened, and ground in a cyanide solution in tube mills passing through a 200-mesh screen. From the tube mills it passed into separation boxes. These boxes were from 5 to 7 feet deep and conical in shape and the finely ground slimes would rise through the cyanide solution, (two pounds of cyanide to one ton of water) and then overflow. Coarse material would be returned to the mills and the process repeated. When it was sufficiently ground, lime or lime water was added to coagulate the slimes and make the mix easier to settle in the settling tanks. From the bottom of these tanks the thick slime was pumped into air agitation tanks. When the bullion was dissolved the sludge was drawn off for the vacuum filter which would extract the cyanide solution, and wash the slime before it was discharged. This was dumped on the ground and with a trickle of water it was washed back into the river. It went in just above the dam of the Waihi Mining Company. All the slimes and thousands of tons of tailings from two miles upstream were dumped back into the river just above this dam.

During 1909 a full year of operation 23,950 tons of tailings had been treated with thirty men employed. This new syndicate was so delighted that on the 4 March 1910 it called for a public company to be formed which called itself The Waihi Paeroa Gold Extraction Company Limited. It was also in 1910 that a start had been made in Paeroa to erect a new plant off Mill Road.

Mr. Aitken in a letter to Mr M. Paul the Inspector of Mines said "the average number of employees during the year of 1910 had been thirty but work time was down owing to a heavy rainfall that caused the river to rise by twenty feet." Even with this set back he said, "the Waihi plant had managed to treat 7100 tons of tailings." March 31 1910 was the last financial year of operation at the Waihi site for the company. This plant had treated a total of 32,800 tons of tailings for £14,000 and the company was full of confidence that the improved appliances for lifting and treating tailings had and would continue to prove more than satisfactory.

Extensive alterations had been made so that the new machinery would be able to double the capacity of the old plant. The Paeroa Plant would be equipped with every "modern" convenience necessary for the operations. The company was looking forward to a prosperous operation when their new plant was fully installed and operating. This was the end of the plant at Waihi first known as the Waihi Dredging Company. At the end of April came the physical demise of the original Ohinemuri River Syndicate site when tenders were advertised for the dismantling of the Waihi plant and have it removed to the new site at Paeroa. The new Waihi Paeroa Gold Extraction Plant off Mill Road, Paeroa was in the process of being completed and its place in history for dredging tailings for gold was near.